After monochromatic light is data modulated, the frequency spectrum of the light is broadened, and different frequency components have different transmission speeds, causing that light with different frequencies arrive at different time, i.e., chromatic dispersion effect. In an optical fiber digital communication system, chromatic dispersion effect causes an optical pulse to be broadened, thereby limiting the highest rate the system can transmit. Modulation format and symbol rate of an optical signal both affect the chromatic dispersion that an optical communication can tolerate very greatly. For example, for a 10 Gbps optical fiber communication system, the non-compensation transmission distance of a standard single mode optical fiber is about 60 km, and a transmission distance for a 40 Gbps system is only 3.5 km. Therefore, in the optical fiber communication system and network, it is necessary to monitor in real-time and equalize the chromatic dispersion of the optical fiber link. Traditional methods for measuring an optical fiber chromatic dispersion mainly include a frequency spectrum analysis method, a vestigial sideband filter method, a non-linear spectral analysis method and the like.
The frequency spectrum analysis method mainly includes a radio frequency (RF) pilot method and a clock frequency method. In the RF pilot method, an RF pilot is used as a monitoring object, and the frequency spectrum width of an RF pilot is narrower than that of an optical signal is added into a transmitter. In the optical fiber transmission, a pilot goes through the same chromatic dispersion as the signal. In the case without chromatic dispersion, upper and lower sidebands of the pilot are of the same phase, and the reception power is maximum; when there is chromatic dispersion, the upper and lower sidebands of the pilot have a phase difference, attenuation of RF power occurs at a receiver end, and chromatic dispersion is measured by measuring the pilot power. If the pilot frequency is high, the sensitivity of monitoring is high, but the range that can be monitored becomes small; conversely, the range to be monitored is large, and the sensitivity is low, constituting a pair of contradictions. Loading both a high-frequency pilot and a low-frequency pilot can improve the dynamic range and sensitivity of chromatic dispersion monitoring. Chromatic dispersion monitoring with pilot modulation has limitations in that it is impossible to distinguish between positive or negative chromatic dispersion, and it is necessary to change the transmitter. The clock frequency method measures chromatic dispersion by extracting the clock frequency component of an optical signal to be measured and monitoring the change of the clock frequency spectrum power. As for the measurement of optical fiber chromatic dispersion by extracting a clock frequency component, although it is not necessary to change the transmitter, it is still impossible to distinguish between positive or negative chromatic dispersion.
The vestigial sideband filter method uses a band-pass filter to filter upper and lower sidebands of the optical signal, and can measure chromatic dispersion of an optical fiber by detecting a time delay difference between two vestigial sideband signals. After optical-electric conversion, phases of the two vestigial sideband signals are detected to obtain a phase difference between the two signals, thereby indirectly measuring the optical fiber chromatic dispersion. In the vestigial sideband filter method, it is not necessary to change the transmitter, the sensitivity is high, positive or negative chromatic dispersion can be distinguished, and is free from polarization mode chromatic dispersion, non-linear birefraction and chirp. However, the bandwidth of the band-pass filter is equal to a bit rate of the signal, and is opaque to the signal rate.
Principle of the non-linear spectral analysis method is that: chromatic dispersion causes the broadening to the time domain pulse of an optical signal to be broadened and reduces the peak power, making the non-linear effect caused by the optical fiber be reduced, thus the optical power passing through the band-pass filter will be reduced, therefore, change of chromatic dispersion can be monitored by measuring the change of the optical power subsequent to the filter. The non-linear spectral analysis method has the limitation that it is impossible to distinguish positive and negative chromatic dispersions, and the interaction between the chromatic dispersion and non-linearity may affect the accuracy of chromatic dispersion monitoring.
The above-described methods have respective defects, thus it is impossible to perform a chromatic dispersion monitoring without interference and with transparent modulation format and transparent signal rate. Therefore, it is a current difficulty that urgently needs to be solved as for how to accurately test chromatic dispersion of an ultra-high speed optical fiber communication link in real time without interference.